[1] Two case studies are performed to investigate substorm timing and activations based on Double Star TC1, Cluster, Polar, IMAGE, LANL geostationary satellites and ground-based geomagnetic field measurements. In both events, an earthward flow associated with plasma sheet thinning is measured by Cluster 8-10 min ahead of the auroral breakup. A couple of minutes after the breakup, either TC1 at $X-10 R E first detects plasma sheet expansion and then the LANL satellites near the midnight measure energetic electron injections at geostationary orbit or the LANL satellites first measure the electron injections and then TC1 detects the plasma sheet expansion. More than about 20 min later, Cluster at X$16 R E and Polar (at higher latitude) successively observe plasma sheet expansion. The open magnetic flux of the polar cap, Y, is found to continually increase during the early substorm phase and then to rapidly fall when the IMF turns northward. When Y reaches its minimum value, bright and broad auroral activities start to decrease. Tailward progression of the magnetic dipolarization and a poleward expansion of auroral bulges are shown to closely map to one another. These results suggest that substorm activations start in the midtail before ground onset and then move earthward, which leads to an expansion onset in the near-Earth tail around X$ -(8-9) R E . After onset, the activations progress both earthward and tailward. Substorm onset is possibly related to plasma sheet reconnection of close field lines, while tail lobe reconnection of open field lines release more energy to support the full expansion of the substorm. In a fully developed expansion phase, an initial dipolarization in the near-Earth may eventually evolve to enable disruption of the cross-tail current over a wide region of the magnetotail. Citation: Cao, X., et al. (2008), Multispacecraft and ground-based observations of substorm timing and activations: Two case studies,

We use three-dimensional particle-in-cell simulations to demonstrate that a plasma density channel can stably guide the petawatt laser pulse in near critical plasmas. In this regime, a directed, collimated, and micro-sized gamma photon beam is emitted by the direct-laser accelerated electrons along the channel axis. While in the case without the plasma density channel, the laser tilting behavior leads to the generation of randomly deflected gamma photon beams with a large divergence angle and transverse source size. In addition, in the plasma density channels, the divergence angle of the gamma photon beams can be much reduced by using a smaller value of n0/a0nc. The energy conversion efficiency can also be improved by increasing the laser power or the plasma density. This regime provides an efficient and compact approach for the production of high quality gamma photon beams.

Energetic electron acceleration processes in a plasma hollow tube irradiated by an ultraintense laser pulse are investigated. It is found that the longitudinal component of the laser field is much enhanced when a linear polarized Gaussian laser pulse propagates through the plasma tube. This longitudinal field is of π/2 phase shift relative to the transverse electric field and has a π phase interval between its upper and lower parts. The electrons in the plasma tube are first pulled out by the transverse electric field and then trapped by the longitudinal electric field. The trapped electrons can further be accelerated to higher energy in the presence of the longitudinal electric field. This acceleration mechanism is clearly illustrated by both particle-in-cell simulations and single particle modelings.

The dispersion relation of one-dimensional longitudinal plasma waves in relativistic homogeneous plasmas is investigated with both linear theory and Vlasov simulation in this paper. From the Vlasov-Poisson equations, the linear dispersion relation is derived for the proper one-dimensional J€ uttner distribution. Numerically obtained linear dispersion relation as well as an approximate formula for plasma wave frequency in the long wavelength limit is given. The dispersion of longitudinal wave is also simulated with a relativistic Vlasov code. The real and imaginary parts of dispersion relation are well studied by varying wave number and plasma temperature. Simulation results are in agreement with established linear theory. V C 2013 AIP Publishing LLC.

The hosing processes of a relativistic laser pulse, electron acceleration, and betatron radiation in a parabolic plasma channel are investigated in the direct laser acceleration regime. It is shown that the laser hosing instability would result in the generation of a randomly directed off-axis electron beam and radiation source with a large divergence angle. While employing a preformed parabolic plasma channel, the restoring force provided by the plasma channel would correct the perturbed laser wave front and thus suppress the hosing instability. As a result, the accelerated electron beam and the emitted photons are well guided and concentrated along the channel axis. The employment of a proper plasma density channel can stably guide the relativistically intense laser pulse and greatly improve the properties of the electron beam and radiation source. This scheme is of great interest for the generation of high quality electron beams and radiation sources.

It was shown that in the interactions of ultra-intense circularly polarized laser pulse with the near-critical plasmas, the angular momentum can be transferred efficiently from the laser beam to electrons through the resonance acceleration process. The transferred angular momentum increases almost linearly with the acceleration time t_{a} when the electrons are resonantly accelerated by the laser field. In addition, it is shown analytically that the averaged angular momentum of electrons is proportional to the laser amplitude a_{L}, and the total angular momentum of the accelerated electron beam is proportional to the square of the laser amplitude a_{L}^{2} for a fixed parameter of n_{e}/n_{c}a_{L}. These results are verified by three-dimensional particle-in-cell simulations. This regime provides an efficient and compact alternative for the production of high angular momentum electron beams, which may have many potential applications in condensed-matter spectroscopy, new electron microscopes, and bright x-ray vortex generation.

Filamentation dynamics of relativistic optical vortex beams (OVBs) propagating in underdense plasma is investigated. It is shown that OVBs with finite orbital angular momentum (OAM) exhibit much more robust propagation behavior than the standard Gaussian beam. In fact, the growth rate of the azimuthal modulational instability decreases rapidly with increase of the OVB topological charge. Thus, relativistic OVBs can maintain their profiles for significantly longer distances in an underdense plasma before filamentation occurs. It is also found that an OVB would then break up into regular filament patterns due to conservation of the OAM, in contrast to a Gaussian laser beam, which in general experiences random filamentation.

Dimensional effects in particle-in-cell (PIC) simulation of target normal sheath acceleration (TNSA) of protons are considered. As the spatial divergence of the laseraccelerated hot sheath electrons and the resulting space-charge electric field on the target backside depend on the spatial dimension, the maximum energy of the accelerated protons obtained from three-dimensional (3D) simulations is usually much less that from two-dimensional (2D) simulations. By closely examining the TNSA of protons in 2D and 3D PIC simulations, we deduce an empirical ratio between the maximum proton energies obtained from the 2D and 3D simulations. This ratio may be useful for estimating the maximum proton energy in realistic (3D) TNSA from the results of the corresponding 2D simulation. It is also shown that the scaling law also applies to TNSA from structured targets.